Diisocyanates are a known useful class of compounds. They are reactive with active hydrogen-containing compounds such as polyhydroxy compounds to produce polyurethanes and polyamido compounds to produce polyureas. Such reaction products are useful for protective and decorative coatings, for making molded and extruded articles, such as surgical tubings, and for many other purposes.
Special mention is made of a particularly useful family of diisocyanates: the 3-(isocyanatomethyl)-3,5,5-tri-lower-alkyl cyclohexylisocyanates. These are distinguished by their excellent applicability for cross-linking reactions of proteinaceous materials (tanning agents), and further for example by their superior adhesive properties in the bonding of metals with high-molecular weight synthetic or natural substances as, for example, the bonding of metals with high-molecular weight synthetic or natural substances as, for example, the bonding of metals to various types of rubbers. They are particularly well suited for use in reaction injection molding (RIM) compositions. A diisocyanate particularly effective carries methyl substituents in the 3-, 3- and 5-positions and is known as isophorone diisocyanate (IPDI).
IPDI (isophorone diisocyanate) has been prepared by a number of routes.
In Schmitt et al., U.S. Pat. No. 3,401,190, isophorone diamine (IPDA), also known as 3-(aminomethyl)-3,5,5-tri-methylcyclohexylamine-(1), is reacted with phosgene at 100.degree. C. for 28 hours and the by-product hydrogen chloride and excess phosgene are removed by blowing nitrogen through the liquid reaction mixture, then the product is recovered by vacuum distillation. Drawbacks of this process are the toxicity of the phosgene and corrosion problems associated with by-product HCl.
The difficulties with direct phosgenations have led to the development of non-phosgenation routes, and these generally involve the pyrolytic thermolysis or cracking of a carbamic acid ester of the formula: EQU R.sup.1 (NH--CO--OR.sup.2).sub.n
in which R.sup.1 is an aliphatic or aromatic radical of the type obtained by removal of the isocyanate groups from, for example isophorone diisocyanate (IPDI) or 2,4' and/or 4,4-diisocyanato diphenyl methane (MDI), R.sup.2 is usually a primary or secondary aliphatic hydrocarbon radical of from 1 to 4 carbon atoms and n is 2 or an integer of greater than 2.
In general, the prior art cracking reactions are carried out in the vapor phase or in the liquid phase, with or without accelerators, or catalysts or other additives, generally called auxiliaries. It is a general rule also that both aromatic isocyanates (such as MDI) and aliphatic isocyanates (such as IPDI) are produced under the same general conditions and influenced in the same way by the modifiers and/or promoters employed and, therefore any catalyst useful for one would be expected to be useful for the other.
Merely by way of illustration, Merger et al., U.S. Pat. No. 4,482,499 disclose that urethanes, including IPDU, will be thermally cleaved to isocyanates at temperatures in the range of 175.degree. C. to 600.degree. C. in the presence of carbon, preferably in an agitated carbon bed or in a fluidized bed containing carbon. The drawbacks of this basically gas phase process are that it requires high energy input, produces a lower product output per unit operation time, gives a low IPDU conversion and a moderate IPDI yield, and requires a high b.p. solvent for separation of product. Hellbach et al., European Patent Application No. 126300 A1 (Apr. 17, 1984) prepared IPDI by passing IPDU through a cracker which was packed with brass rings at an average temperature of 410.degree. C. This is considered to be in a gaseous phase, and the drawbacks include the high temperature (high energy) operation, and lower product output per operation time unit. Engbert et al., European Patent Application No. 0092738A1 (Apr. 14, 1983) prepared IPDI and other diisocyanates by cracking IPDU and other carbamic acid esters in a liquid phase, continuous process at a temperature of 150.degree.-450.degree. C., pressure 0.01-20 bar, using a soluble catalyst, e.g., dibutyltin dilaurate, and a high b.p. chlorinated solvent. Drawbacks of this process include a low IPDU conversion and low IPDI yields, requiring a high b.p. solvent as a medium, potential need for make-up for solvent loss, and potential hazards of using chlorinated aromatics as solvent. Sunderman et al., U.S. Pat. No. 4,388,246, prepared IPDI and other diisocyanates from IPDU and other carbamic acid esters by cracking in a liquid phase at a temperature of 150.degree.-350.degree. C. and a pressure of 0.001-20 bar in the presence of auxiliary agents (HCl, organic acid chlorides, organotin (IV) chlorides, etc.). Solvents and catalysts may also be employed. Drawbacks include the requirements that the auxiliary agents are basically chlorides, which may contaminate the final isocyanate product, the loss and make-up of the auxiliary agents which is costly, and the fact chlorides are acidic and therefore cause serious corrosion problems. Moreover, the high vacuum, approximately 4 mm Hg, cannot easily be achieved in industrial operations. Spohn, U.K. Patent Publication No. 2,113,673A (Aug. 10, 1983) describes the production of aromatic isocyanates (not aliphatic) by liquid thermolysis at atmospheric pressure or above (not sub-atmospheric) in the presence of a catalyst containing Ti, Sb, Zr or Sn. The organic soluble tin compounds employed did not include tin oxide. Mitsubishi, Japanese Patent Pubication No. 54-88201 (July 13, 1979) discloses the use of compounds from alkaline earth metals (Be, Mg, Ca, Ba, Sr, Ra) for cracking of urethanes to isocyanates. No mention is made of SnO.sub.2, CuO as cracking catalysts. Moreover, experiments have shown that oxides of alkaline earth metals (BeO, MgO, CaO, SrO, RaO) give low IPDI yields (see infra). The only examples in this Japanese publication make toluenediisocyanate (TDI), an aromatic compound, and, in general, only low TDI yields were obtained Asahi Kasei, Japanese Patent Publication 57-158747 (Sept. 30, 1982) discloses the use of numerous metal oxides (Zn, Cu, Au, Ag, Cd, Al, Ga, In, Ge, Ti, Zr, Sn, etc.) as cracking catalysts. Also mentioned, but not exemplified, is the use of IPDU and other aliphatic urethanes as substrates. It is claimed that all metal oxides are effective cracking catalysts. In the examples, however, only aromatic urethanes, e.g., MDU are employed, and many of the oxides used are better than SnO.sub.2 and CuO in terms of MDI yields obtained.
It has now unexpectedly been discovered and is the subject matter of this invention that SnO.sub.2 and CuO are clearly superior to all other known metal oxide catalysts when used for the thermal pyrolysis of IPDU to IPDI. In the present invention, SnO.sub.2 and CuO as catalysts give high IPDI yields. In contrast, use of other metal oxides, many of which are substantially superior to SnO.sub.2 and CuO when used with an aromatic substrate, e.g., MDI, actually give lower IPDI yields (5-55%).
The present invention is based on the discovery of two highly specific and selective metal oxide catalysts, SnO.sub.2 and CuO, from many metal oxide catalysts recommended in the prior art. A systematic study has found that these two catalysts are especially attractive for converting isophorone diurethanes to isophorone diisocyanate because they afford IPDI in high selectivity and yield little residual undesired products such as isocyanurate, carbodiimide and other side products. They are suitable for low temperature, liquid phase processes. Low catalyst loadings are required The catalysts are easily recycled, and there are no specific requirements either for solvents or auxiliary agents, as in some of the prior art. Sub-atmospheric pressures (30-45 mm Hg) are suitable, and these can be easily generated by conventional aspiration equipment. Savings on reactor size are achievable because solvent is not required The catalysts, which are insoluble solids, are easily separated from the products. Although the invention is primarily directed to cracking IPDU (methyl or butyl carbamates) to IPDI, the technology is extendable to other pyrolytic cracking reactions involving alkyl carbamic acid esters attached to aliphatic and cycloaliphatic substituents, for example, the cracking of .alpha.,.alpha.,.alpha.',.alpha.'-tetramethylxylylene diurethane (TMXDU) methyl or butyl ester to .alpha.,.alpha., .alpha.', .alpha.'-tetramethylxylylene diisocyanate (TMXDI).